1. Field of the Invention
This invention relates generally to optical interconnections and more specifically to cost-effective side-coupling interconnections using polymer fiber optics.
2. Discussion of the Prior Art
Market pressure has led optical fiber companies to recognize that polymer optical fibers (POFs) could be a promising cost-effective physical layer networking solution of the future. Recent research and development indicates that some large core ( greater than 0.5 mm) POF solutions can deliver Gbit/s optical data over 100 meter distance. See, for example, Y. Koike et al. (1995) xe2x80x9cHigh-bandwidth graded-index polymer optical fiberxe2x80x9d IEEE J. Lightwave Technology 13: 1475-1489 and S. Yamazaki et al. (1996) xe2x80x9cA 2.5 Gb/s 100 m GRIN plastic optical fiber data link at 650 nm wavelengthxe2x80x9d Graded Index POF Boston: Information Gatekeepers: 98-101. Research is also being conducted to identify methods to fabricate low-cost and low-loss POF""s for long wavelengths. See, for example, T. Ishgure et al. (1994) xe2x80x9cLarge-core high-bandwidth polymer optical fiber and its applicationsxe2x80x9d Technical Digest of CLEO/EUROPE ""94: paper CThD5. The POF technology is expected to provide the following significant cost advantages over its glass optical fiber (GOF) counterparts in many areas from raw material cost to processing and connection costs.
Due to a high melting temperature and rigidity, it is difficult (although possible in principle) to etch structures on glass materials. Research has been performed to etch a single micro-mirror into a conventional GOF. See, for example, D. J. Ripin et al. (1995) xe2x80x9cHigh efficiency side-coupling of light into optical fibers using imbedded v-groovesxe2x80x9d Electron. Lett. 31: 2204-2205. However, the corresponding low-cost volume production process is not available. In the past, optical couplers coupling light into or out of a GOF were prefabricated in a controlled environment. Methods of using prisms or blazing gratings attached to fibers, using evanescent waves and using electromagnetic mode coupling concepts are the most popular light coupling approaches.
The coupling mechanism of a sequence of unidirectional, mirror-based side couplers can be modeled using the geometry of FIG. 1(a). Here the two sets of marked parameters are: xcex1i (1xe2x89xa6ixe2x89xa6N) which are mirror reflective coefficients and xcex3i which are receiving fiber""s transmission coefficients at outputs, both are smaller than or equal to unity. For limited POF length, its absorption and scattering can be omitted. Letting Pi be the output power at the ith port, it can be shown that the receiving power at the N receiving ports are:
xe2x80x83P1=xcex11xcex31Pin,
P2=(1xe2x88x92xcex11)xcex12xcex32Pin,xe2x80x83xe2x80x83(1)
. . . 
PN=(1xe2x88x92xcex11)(1xe2x88x92xcex12) . . . (1xe2x88x92xcex1Nxe2x88x921)xcex1Nxcex3NPin.
A uniform power distribution to N ports implies that:                                                                                           γ                  2                                ⁢                                  α                  2                                                            γ                1                                      =                                          α                1                                            1                -                                  α                  1                                                              ,                      
                    ⁢          …                ⁢                  
                ⁢                                                            γ                N                            ⁢                              α                N                                                    γ                              N                -                1                                              =                                    α                              N                -                1                                                    1              -                              α                                  N                  -                  1                                                                                        (        2        )            
The residue output power Pout is defined as
Pout=(1xe2x88x92xcex11)(1xe2x88x92xcex12) . . . (1xe2x88x92xcex1N)Pinxe2x80x83xe2x80x83(3)
Using Equation (2), we have                               P          out                =                                                            α                1                            ⁢                              γ                1                                                                    α                N                            ⁢                              γ                N                                              ⁢                      (                          1              -                              α                N                                      )                    ⁢                      P                          i              ⁢                              xe2x80x83                            ⁢              n                                                          (        4        )            
There exist at least two ways to solve for xcex1""s and xcex3""s. The first case is the constant-xcex3 case, which requires xcex31=xcex32=. . . =xcex3N=xcex3xe2x89xa61 The simplest possible situation is that xcex3=1, which corresponds to either a perfect coupling or a situation where light is coupled to free-space. The coefficients xcex1 can then be evaluated as:                                                         α              1                        =                                                            P                                      i                    ⁢                                          xe2x80x83                                        ⁢                    n                                                  -                                  P                  out                                                            NP                                  i                  ⁢                                      xe2x80x83                                    ⁢                  n                                                              ,                      
                    ⁢                                    α              2                        =                                          α                1                                            1                -                                  α                  1                                                              ,                      
                    ⁢          …                ⁢                  
                ⁢                                                                              α                  N                                =                                  xe2x80x83                                ⁢                                  α                  1                                                                                                                          xe2x80x83                                ⁢                                  1                  -                                                            (                                              N                        -                        1                                            )                                        ⁢                                                                  α                        1                                            .                                                                                                                              (        5        )            
Correspondingly, the power outputs are:                               P          1                =                              P            2                    =                      …            =                                          P                N                            =                                                                    P                                          i                      ⁢                                              xe2x80x83                                            ⁢                      n                                                        -                                      P                    out                                                  N                                                                        (        6        )            
For example, let Pout=0, we then have xcex11=1/N, xcex12=1/(Nxe2x88x921), . . . xcex1Nxe2x88x921=xc2xd, and xcex1N=1.
The second case is relevant for fiber-to-fiber coupling where receiving coefficients xcex3s may not be identical. In this case, by letting xcex11=xcex12=. . . xcex1N=xcex1 in a so-called constant a situation, the relation between input and output power is:                     α        =                  1          -                                    [                                                P                  out                                                  P                                      i                    ⁢                                          xe2x80x83                                        ⁢                    n                                                              ]                        ⁢                          1              /              N                                                          (        7        )                        and                      xe2x80x83                                                      γ            2                                γ            1                          =                                            γ              3                                      γ              2                                =                      …            =                                                            γ                  N                                                  γ                                      N                    -                    1                                                              =                              1                                  1                  -                  α                                                                                        (8a)                        or                      xe2x80x83                                                      γ            j                                γ            i                          =                              (                          1                              1                -                α                                      )                                j            -            i                                              (8b)            
where j, like i, is a port index and where j greater than i to guarantee the validity of equation 8(b). In the most efficient coupling case, let xcex31=1. We then have:                               P          1                =                              P            2                    =                      …            =                                          P                N                            =                                                                                          [                                                                        P                          out                                                                          P                                                      i                            ⁢                                                          xe2x80x83                                                        ⁢                            n                                                                                              ]                                                                                      (                                                  N                          -                          1                                                )                                            /                      N                                                        ⁡                                      [                                          1                      -                                                                                                    (                                                                                          P                                out                                                                                            P                                                                  i                                  ⁢                                                                      xe2x80x83                                                                    ⁢                                  n                                                                                                                                              "AutoRightMatch"                                                                          1                          /                          N                                                                                      ]                                                  ⁢                                  P                                      i                    ⁢                                          xe2x80x83                                        ⁢                    n                                                                                                          (        9        )            
Although the balanced power is reached, this scheme requires a non-zero Pout/Pin and thereby leads to inefficient usage of power. For example, for a pre-determined ratio Pout/Pin, Equation (7) first yields an xcex1 value. Using Equation (8), xcex3 values can then be determined as xcex3i=(1xe2x88x92a)Nxe2x88x92i, for 1xe2x89xa6ixe2x89xa6N. The most general case is when neither xcex1 nor xcex3 is a constant. Equations (1) and (2) have to be used to calculate for each coupler. Since there are 2N constants to be decided, a general procedure is for a required PN, first determine all 0xe2x89xa6xcex1ixe2x89xa61 (1xe2x89xa6ixe2x89xa6N) and xcex3N using the last equation in Equation(1). Corresponding to the set of N+1 constants, the remaining xcex3i(1xe2x89xa6ixe2x89xa6Nxe2x88x921) can be calculated using Equation (2). As a numerical example, letting Pout=0.1 Pin and N=16, we have calculated both constant-xcex1 and constant-xcex3 cases and plotted the parameters xcex1, xcex3, xcex7=Pi/Pin in FIG. 1(b) where xcex7is defined as the power ratio between the ith port and the input. Squares and triangles denote the curves for constant xcex1 and constant xcex3, respectively. It can be seen that since xcex3 could be as large as unity, the overall coupling efficiency for the constant-xcex3 case is, in general, larger than that of the constant-xcex1 case.
Couplings between POF and Free-space: One simple but useful application of using micro-mirrors along a POF is to deliver equal intensity optical signals to N different locations along a fiber. Previous methods of delivering optical signals were reported using either holograms, stacked birefringent crystals and integrated optical wave-guides. See, for example, J. W. Goodman et al. (1980) xe2x80x9cOptical inter-connections for VLSI systemsxe2x80x9d Proc. IEEE 72: 850-858; R. T. Chen et al. (1992) xe2x80x9cGuided-wave planar optical inter-connects using highly multiplexed polymer waveguide hologramsxe2x80x9d J. Lightwave Tech. 10: 888-897; J. Jahns (1994) xe2x80x9cDiffractive optical elements for optical computersxe2x80x9d Optical Computing Hardware, eds. J. Jahns and S. H. Lee, New York: Academic Press: 137-167; and T. W. Stone et al. (1994) xe2x80x9cOptical array generation and Interconnection using birefringent slabsxe2x80x9d Appl. Opt. 33: 182-191. The availability of a POF splitter can provide a flexible yet low-cost method and apparatus for delivering optical signals to receiving terminals with flexible spacings than the above mentioned situations. See, for example, Y. Li et al. (1996) xe2x80x9c4xc3x9716 polymer optical fiber array couplersxe2x80x9d IEEE Photon. Tech. Lett. 8: 1650-1652.
It is, therefore, an object of the present invention to provide a low cost optical fiber coupler apparatus and method for fabrication thereof.
It is another object of the present invention to provide a flexible optical fiber coupler apparatus and method for fabrication thereof.
It is yet another object of the present invention to provide an optical fiber coupler apparatus and method for fabrication thereof in which the highly flexible mechanical processing capability of POFs are utilized.
The availability of POFs makes it possible to incorporate, in addition to the above mentioned coupling approaches, a scheme to-use simple optical reflection for couplings. See, for example, U.S. Pat. No. 4,872,739, to Kahn et al.; B. P. Keyworth et al. (1995) xe2x80x9cDistributed serial taps in dispensed polymer multimode waveguidesxe2x80x9d 1995 OSA Annual Meeting, Sep. 10-15, 1995 (Portland, Oreg.) talk WVV52; U.S. Pat. No. 5,432,876, to Appeldorn et al.; Y. S. Liu et. al. (1996) xe2x80x9cPolymer optical interconnect technology (POINT): optoelectronic packaging and interconnect for board and backplane applicationsxe2x80x9d Optoelectronic Interconnects and Packaging (eds. R. T. Chen and P. S. Guilfoyle) SPIE CR62: 405-414; allowed U.S. patent application Ser. No. 08/ADD SERIAL NO., to Li et al. entitled xe2x80x9cOptical Fiber Tapsxe2x80x9d; and Y. Li et al. (1996) xe2x80x9cDistribution of light and optical signals using embedded mirrors inside polymer optical fibersxe2x80x9d IEEE Photo. Tech. Lett. 8: 1352-1354. Both the conventional reflection and the total internal reflection can easily be implemented through mirrors formed inside a POF core using mechanical or other types of low-cost, in-print etching techniques. Using a polished knife, a relatively smooth surface can be etched in different shapes and angles on a POF. A mirror can be formed either by satisfying the total internal reflection condition of the etched interface or by metal deposition using a conventional metallic mirror coating process. In the later case, after a mirror is formed, it is possible to re-deposit the polymer to embed the cut so that the mirror is physically inside the POF. One obvious advantage of using such mirrors to couple light into or our of a fiber is that it offers a wide range of flexible coupling directions in three-dimensional (3D) space.
Accordingly, a first embodiment of the optical couplers of the present invention is provided wherein the optical fiber coupler is configured to be unidirectional. The unidirectional coupler comprises at least two input polymer optical fibers, each having a central axis for transmission of an input optical signal therealong and at least two output polymer optical fibers, each also having a central axis for transmission of an output optical signal therealong. The output optical fibers are positioned substantially perpendicular to the input optical fibers forming a grid of optical fibers. Also provided is a body in which the input and output optical fibers are housed, with each input optical fiber being in optical communication with each output optical fiber at each point where the input and output optical fibers cross. An input mirror is formed within each input optical fiber and positioned at each point of crossing for reflecting inputted optical signals substantially perpendicular to their axial direction and into its corresponding output optical fiber. Each input mirror comprises a notch having at least one surface angled with respect to the central axis of the optical fiber for reflecting an optical signal perpendicular to its central axis. Similarly, an output mirror is formed within each output optical fiber and positioned at each point of crossing for reflecting the perpendicular optical signal from its corresponding input optical fiber to a direction along its central axis. Each output mirror comprises a notch having at least one surface angled with respect to the central axis of the optical fiber for reflecting the perpendicular optical signal from its corresponding input optical fiber parallel to its central axis. The apparatus provides for coupling of the input optical signals with any combination of the input optical fibers and transmitted into any combination of the output optical fibers.
A second embodiment is also provided which is configured as a bidirectional optical fiber coupler. The bidirectional coupler comprises a first polymer optical fiber having a central axis for transmission of optical signals therealong and at least two tapping polymer optical fibers, each having a central axis for transmission of an optical signal therealong. The. tapping optical fibers are positioned substantially perpendicular to the input optical fibers and each terminate at the first optical fiber at a side coupling. Also provided is are bidirectional mirrors formed within the first optical fiber and positioned at each side coupling for reflecting optical signals from the tapping optical fiber substantially perpendicular to its axial direction and into the first optical fiber. Each mirror is centered along the central axis of its corresponding tapping fiber and comprises a notch having two surfaces angled with respect to the central axis of the first optical fiber for reflecting an optical signal perpendicular to its central axis. The configuration of the second embodiment allows optical signals from the tapping fibers to be reflected in opposite directions along the first optical fiber.
Another aspect of the present invention is an apparatus and method for fabricating the inventive couplers disclosed herein.